Radio tuning indication



Nov. 15, 1938. A. w. BARBER- 2,136,665

RADIO TUNING INDICATION Filed Feb. 1 8, 1956 FESRESELECTOR AUDIO L AND RADIO FREQ. DETECTOR AMPLIFIER AMPLIFIER.

Ii SPEAKER 7 SPECIAL DIRECT 59.1. DETECTOR CURRENT? AMPLIFIER LAMP 11v VENTOR Patented Nov. 15, 1938 UNITED STATES PATENT ()FFICE RADIO TUNING INDICATION Alfred W. Barber, Flushing, N. Y.

Application February 18, 1936, Serial No. 64,517 4' Claims. (oi. 250--4o This present invention of mine concerns provements in tuning indicator methods means, adapted for use in radio receivers the like.

imand and One object of my invention is to provide a tuning indicator which operates to indicate correct tuning in radio receivers and the like.

of the signal being tuned in. Still another ob ject is to provide a tuning indicator which operates by means of a lamp which lights when a signal is correctly tuned in and operates by automatic electrical means.

In continuously tunable radio receivers ploying automatic volume control, the point of exact tuning is difficult to locate by ear, due to the fact that in passing thru a signal the automatic 'volume control system follows the signal variations tending to keep the loud speaker output constant. In order to make accurate tunin possible a number of different indicators have been devised. The most Widely used indicators are meters or lamps actuated by plate current changes in the amplifier tubes controlled by the Correct tuning is indicated by a minimum amplifier tube plate current or maximum plate voltage due to the ac tion of the automatic volume control voltage. These systems operate much better on strong signals than on weak ones and may even ceaseto operate on signals below a certain level. The apparent sharpness of the indication also varies with signal strength, since moving say kilocycles on a strong station may actuate an indicator much further than 5 kilocycles change on a, Weaker station.

In my copending application for Letters Patent entitled Rectifier circuits filed January 30, 1

and bearing Serial No. 61,457 I have shown a rectifier circuit having some unique properties. This rectifier circuit is used in my present tuning indicator system.

My present system of tuning indication overcomes the difficulties pointed out above since the indicator flashes on at correct tuning giving a Very sharp indication and operates with equal sharpness on strong or weak signals.

I employ two diode circuits rectifying the carrier frequency output of the radio receiver.

One diode circuit actuate a lamp or other indicating device.

operation the long time constant diode load charges up as the receiver is tuned thru a signal to a maximum voltage point the value of which depends on the signal strength. Since the time constant is large, the circuit holds this voltage. 5 as the receiver is tuned beyond the signal. On the other hand, the short time constant circuit charges up with the signal but discharges again as the signal is tuned out again. Thus one voltage indicates the maximum signal strength and the other indicates the instantaneous'signal or tuning point. If an indicator is attached which operates when both voltages are equal, the indicator will show exact tuning as exact tuning exists only when both diode voltages are equal at maximum response.

The appended claims set forth, in particular, the novelfeatures to be found in this invention.

The following description, however, when taken in connection with the drawing, will serve to set forth the theory and mode of operation of my invention.

In the drawing- Fig. 1 shows 'a'block diagram of the general form of my invention.

Fig. 2 shows a circuit of an indicating system embodying my invention.

Fig. 3 shows a circuit equivalent to one mode of operation of the circuit of Fig. 2.

Fig. 4 shows a circuit equivalent to another mode of operation of the circuit of Fig. 2.

Fig. 5 shows a circuit of another form of my invention.

V Fig. 6 shows curves of voltages generated by various forms of my invention. a

The block diagram shown in Fig. 1 indicates the general form of my tuning indicator system. A conventional automatic volume control receiver system is indicated including a pre-selector and frequency voltage is a maximum at this point.

Fig. 2 shows a circuit diagram of one form of my tuning indicator disengaged from the rest of the receiver. It embodies the double diode rectifi r tube I, which may be two separate diodes as secondary I1.

well, the direct current amplifier tube 2, and the indicating lamp 3 which may be some other type of indicator such as a meter or light shutter. The unrectified voltage is introduced into the diode circuit by means of the transformer having a primary 4 and a magnetically coupled secondary 5. This transformer may be a part of the receiver or may be a branch circuit for the tuning indicator and capacity coupling may be used to coil 5 doing away with the primary 4. The primary will in general be connected in parallel with the regular receiver detector as shown in Fig. 1. The alternating current voltage appearing across coil 5 is impressed on the two diodes, thru the independent load circuits consistingof resistor 6 by-passed by condenser I and resistor 8 bypassed by condenser 9.

input coil 5. A common cathode may be used in place of cathodes I0 and II. The resistor 6 condenser 'I load is in series with coil 5 and diode anode I2 and the resistor 8 condenser 9 load is in series with coil 5 and the other diode anode I3. The diode anode I2 is connected to ground or reference potential point G and the rectified output is taken off between anode I3 ,A steady'grid bias'is impressed on tube 2 by battery E0 connected between cathode I 5 and ground G; The indicator part of the circuit consists of a neon or gas filled glow lamp 3 parallel fed from plate I6 and alternating current source I'I thru variable resistor I8. The alternating current source Il may bethesecondary of a transformer with primary I9 connected to the alternating current power lines. Inoperation the resistor I8 and the plate resistance'of'tube 2 act to divide thevoltage from secondary 11. If the grid I4 is made more positive or less negative the plate resistance of tube 2 decreases There will be a critical biason grid I4 such that lamp 3 just lights on the peaks of the alternating current wave for particular values of resistor I8 and voltage across If EC is made equal to this critical bias and the only voltage placed on grid I4 is positive with respect to ground, lamp 3 will only light when zero voltage is placed on grid I4. The operation of the indicator depends on the production of zero net voltage by the rectifier when the receiver is exactly tuned.

Fig. 3.is useful in explaining the operation of Fig. 2. "If the time constant of resistor 6 shunted by condenser I is made long,,increases in input signal will build up the charge in condenser I but decreases in signal will leave the charge to discharge slowly thru resistor 6. If the circuit of Fig. 2 is attached to a radio receiver and the receiver is tuned thru a signal, the voltage across condenser I will build up to a maximum value Fig. 3 in which the voltage across condenser I is replaced by battery 20 with the negative end grounded. If the time constant of condenser 9 The diode cathodes I0 and II are connected together and to one end of and resistance 8 in parallel is low enough so that the voltage across condenser 9 follows the signal variations due to tuning, the net voltage across points 2| and 22 will indicate the tuning condition. If the tuning is carried to a point where no signal is picked up, the voltage across condenser 9 drops to zero and the point 2I is positive with respect to ground by the voltage of battery 20. Now if the tuning is returned to the signal, as the signal is tuned in the voltage across condenser 9 builds up and the net voltage between point 2I and ground drops. When the signal is again tuned into maximum response, the voltage across condenser!) equals the voltage of battery 20 and the net voltage between point 2I and ground is ZerO. t r

While not limited to these values, I have found that in'ge'neral the time constant of condenser I in parallel with resistor 6 should be greater than one second and the time constant of condenser 9 inpara'llel with resistor 8 should be less than one second for the best operation of my system.

When the actual circuit of Fig. 2 is used, the condenser I charges upon the first excursion thru the signal establishing the reference voltage equivalent to battery 20 and the voltage supplied to grid I4 from the rectifier drops to zero only when the tuning is restored to the point giving maximum response. As explained above the lamp 3 is thus caused to light when no external bias is fed to grid I4 which takes place only at exact tuning of the receiver.

Fig. 6 shows curves of the voltages produced in the rectifier circuits of Fig. 2 upon tuning thru a signal. Curve a represents the steady positive reference potential established in condenser l upon'tuning thru a signal. The difference between curves 0 and b is the voltage across condenser 9 upon tuning back thru the signal and curve I) is the net voltage passed on to grid I4. This shows that this net voltage is zero at h which point is the point of resonance for the tuning system. For the purpose of allowing reasonable tolerance in the system it may be necessary to allow lamp 3 to light whenever the external voltage applied to grid l4 falls below say 0.5 volt. These limits are indicated on Fig. 6 by line e at 0.5 volt or whatever arbitrary voltage is chosen. The intersection of c with b shows that under these conditions lamp 3 will light from g to 7' which may for instance correspond to 1000 cycles either'side of exact tuning.

Fig. 5 shows a circuit in which lamp 3 is connected in series with the amplifier tube plate. The rectifier I is inverted so that the load circuits are in the diode cathode circuits. This reverses the polarity of the rectified voltages with respect to ground when cathode I0 is grounded and the output is taken off between cathode I I and ground G. If, as before, the time constant of resistor 6 and condenser I in parallel is made long, a reference potential diiference c as shown in Fig. 6 will be established across condenser I. Upon tuning thru a signal the net voltage between cathode II and ground G will follow curve (1 and the net voltage will be zero at exact tuning.

With lamp 3 in series with the plate I6 of tube 2 and in parallel with resistor I8, the supply voltage EB plus the alternating current voltage across transformer secondary II divides between resistor I8 and the internal plate resistance of tube 2. If bias E0 is made such that lamp 3 just lights with zero voltage supplied to grid I4 from the rectifier circuit, lamp 3 will be extinguished as soon as the plate resistance is increased, since increasing the plate resistance lowers the voltage drop across the resistor 18 and lamp 3. The plate resistance increases in tuning over curve dexcept at exact tuning. Thus the lighting of the lamp indicates exact tuning.

The rectifier part of Fig. 2 can be reversed making the series lamp connection possible simply by reversing the long and short time constant loads. This is shown in Fig. 4 where the battery 20 equivalent of the long time constant circuit is in series with anode i3 and the resulting voltage is negative except at exact tuning. Similarly the rectifier part of Fig. 5 may be reversed permitting the parallel fed indicating lamp with this rectifier connection.

The various systems may also be set-up so that at a critical bias the lamp goes out and the bias from the rectifier keeps the lamp lighted except at exact tuning. This is accomplished by interchanging the series or parallel lamp connection with the parallel or series connection.

In this case the lamp extinguishes at exact tun In general gas filled glow lamps which I propose as suited to this tuning system require a higher voltage to break down than to extinguish. For instance a lamp may not light until the voltage across it is 100 volts but upon reducing the voltage, the lamp may remain lighted until the drop across it falls to 80 volts. A lamp of this type in my tuning system, with direct current only on the amplifier tube plate, will show a drag in the direction of the tuning due to this difference between ignition and extinction voltage. By using an alternating current Voltage in series with the lamp having an amplitude greater than the difference between ignition and extinction voltages the lamp is turned on and ofi at an invisible rate preventing any drag effect. If the tuning is carried to a point below the ignition condition, the alternating current turns the lamp off and it does not ignite again until the tuning is returned to a favorable point. Thus the visible indication follows the ignition condition curve regardless of the lag or drag eifect in the lamp.

While I have described only a few systems whereby my invention may be carried into effect and have pointed out a few possible variations, it will be apparent to one skilled in the art that many modifications are possible without departing from its spirit and scope as set forth in the appended claims.

What I claim is: I

1. In a carrier wave receiver, the combination of receiver tuning means, a tuning indicator and a rectifier comprising two rectifying circuits, means for applying carrier voltage to said rectifier, means for producing a unidirectional voltage proportional to the maximum amplitude of a carrier voltage applied to said rectifier and means for maintaining said unidirectional voltage substantially constant for a predetermined period of time, means for producing a second unidirectional voltage proportional to the instantaneous amplitude of said carrier voltage applied to said rectifier and means for operating said tuning indicator when the difference between the two said unidirectional voltages is less than a predetermined amount.

2. In a tuneable carrier wave receiver, the combination of a tuning lamp, a thermionic vacuum tube amplifier including a grid, a cathode and a plate and a rectifier including two rectifying circuits, means for applying carrier voltage to said rectifier, means for producing a voltage proportional to the peak unmodulated carrier voltage applied to said rectifier and means for maintaining said voltage substantially constant for a predetermined period of time greater than the normal time required to tune said receiver thru a desired carrier in one rectifying circuit of said rectifier, and means for producing a second voltage proportional to the instantaneous peak unmodulated carrier voltage applied to said rectifier in the other rectifying circuit of said rectifier, means for applying a voltage proportional to the difierence between the two said rectified voltages to the grid'of said amplifier and a series circuit in the plate circuit of said amplifier including said lamp and a source of alternating current whereby said lamp operates when said voltage difference is less than a predetermined amount.

- 3. In a carrier wave receiver, the combination of a gaseous glow lamp, receiver tuning means, a

, thermionic vacuum tube amplifier including at least a grid, a cathode and a plate, and a rectifier including at least two rectifying circuits, means for applying carrier voltage to said rectifier, means for producing a voltage proportional to the amplitude of the carrier applied to said rectifier at receiver resonance with said carrier and means for delaying the decay of this voltage so that its decay lags substantially behind the decay of carrier voltage applied to said rectifier due to tuning said receiver away from said carrier in one rectifying circuit of said rectifier, means for producing a second voltage proportional to the instantaneous amplitude of the carrier applied to said rectifier in a second rectifying circuit of said rectifier, means for applying a voltage proportional to the difference between said two rectified voltages to said grid and means for operating said glow lamp from the plate of said amplifier when the difference between the two said rectified voltages is less than a predetermined amount.

4. In a carrier wave receiver, the combination of a gaseous glow lamp tuning indicator, an amplifier tube including at least a grid, a plate and a cathode and two diode rectifiers, a load circuit having a time constant greater than one second in series with one of said diodes and a load circuit having a time constant of a small fraction of a second in series with the other of said diodes, a connection between one end of each of said load circuits and a common input circuit, a connection between the remaining end of one of said load circuits and ground and a connection between the remaining end of the other of said load circuits and the grid of said amplifier, a connection between said plate and said lamp whereby said lamp glows when said receiver is adjusted to maximum response with a carrier wave.

' ALFREDW. BARBER. 

